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International Food Research Journal 21(1): 165-172 (2014)
Journal homepage: http://www.ifrj.upm.edu.my
Antioxidant and α-glucosidase inhibitory activities of the leaf and stem of
selected traditional medicinal plants
Lee, S. Y., 1Mediani, A., 1Nur Ashikin, A. H, 2Azliana, A. B. S. and 1,2*Abas, F.
1
Department of Food Science, Faculty of Food Science and Technology, Universiti Putra Malaysia, 43400
Serdang, Selangor, Malaysia
2
Laboratory of Natural Products, Institute of Bioscience, Universiti Putra Malaysia, 43400 Serdang, Selangor,
Malaysia
1
Article history
Abstract
Received: 2 July 2013
Received in revised form:
4 September 2013
Accepted: 5 September 2013
The study was aimed to determine the antioxidant and α-glucosidase inhibition activities of
the stem and leaf of five different traditional medicinal plants. The studied plants exhibited
varied antioxidant and α-glucosidase inhibition activities. The antioxidant activities of the
plants were determined through their free radical scavenging capabilities using DPPH assay.
The most potent antioxidant activity was demonstrated by Neptunia oleracea with an IC50 of
35.45 and 29.72 µg/mL for leaf and stem, respectively. For α-glucosidase inhibition activity,
Neptunia oleracea exhibited potential α-glucosidase inhibition activity with IC50 value of
19.09 and 19.74 µg/mL for leaf and stem, respectively. The highest total phenolic content
(TPC) was also marked in Neptunia oleracea leaf and stem with value of 40.88 and 21.21 mg
GAE/g dry weight, respectively. The results also showed that Strobilanthes crispus collected
from two different locations possessed different levels of phenolic content, antioxidant and
α-glucosidase inhibition activities. The study revealed that phenolic compounds could be the
main contributors to the antioxidant and α-glucosidase inhibition activities with R values of 78.9
and 67.4%, respectively. In addition, antioxidant and α-glucosidase were positively correlated
(R = 81.9%). Neptunia oleracea could be suggested as a potential natural source of antioxidant
and antidiabetic compounds that can be used for the prevention or treatment of diabetes.
Keywords
Free radical scavenging
activity
α-glucosidase inhibitory
activity
Traditional medicinal
plants
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Introduction
Free radicals are the molecular species that
possess a single unpaired electron in the outermost
atomic orbit. In spite of different types of free radicals,
those of most concern free radicals in biological
system are the reactive oxygen species (ROS) which
include hydroxyl radical, superoxide anion radical,
hydrogen peroxide, oxygen singlet, hypochlorite,
nitric oxide radical, and peroxynitrite (Valko et al.,
2007). Human body has antioxidant defence system
to cope with the activity of free radicals formed
in the body. However, the imbalance between the
free radical production and the antioxidant defence
arises when the free radicals are overproduced in the
body (Bahorun et al., 2006). This may result in the
accumulation of oxidative stress in the body and cause
damage to macromolecules, such as lipid, protein and
nucleic acid. Oxidative damage to these molecular
species leads to the development of various diseases,
such as cardiovascular disease, cancer, cataract,
diabetes mellitus and Parkinson’s disease (Lobo et
al., 2010; Mayne, 2003).
Diabetes mellitus is a metabolic disease in which
the patient has high level of blood glucose. Generally,
*Corresponding author.
Email: [email protected]
Tel.: +603 89468343; Fax: +603 89423552
there are two types of diabetes mellitus which are type
1 and type 2. Type 1 diabetes is insulin-dependent and
takes place due to the insufficient insulin production,
which arises from defects in the insulin gene. Free
radicals play a significant part in the development of
diabetes mellitus. It is currently hypothesized that
oxidative stress is the common pathogenic factor
contributing to insulin resistance, β-cell dysfunction,
impaired glucose tolerance and ultimately to type
2 diabetes (Ceriello and Motz, 2004). Diabetes in
turn will enhances the oxidative stress in the body
of diabetic patient. The possible mechanisms leading
to enhanced oxidative stress in diabetic patients
include compromised antioxidant defenses, glucose
autoxidation, formation of advanced glycated end
products and a change in the glutathione redox status
(Davison et al., 2002). The enhanced oxidative
stress in diabetes leads to its complication with
cardiovascular disease (Jay et al., 2006). Hence,
consumption of antioxidant rich foods is beneficial in
relieving diabetes particularly the type 2 diabetes.
Other than the oxidative damage, diabetes
also might arise because of the release of glucose
from carbohydrate in the diet resulting in a high
postprandial blood glucose level in diabetic
166
Lee et al./IFRJ 21(1): 165-172
patients. The carbohydrate-hydrolyzing enzyme,
α-glucosidase in the digestive tract will hydrolyzes
the carbohydrate, releasing glucose and cause the
raised postprandial blood glucose level. Hence, the
inhibition of the activity of this enzyme can effectively
reduce the postprandial blood glucose level.
α-Glucosidase inhibitors combine with the intestinal
α-glucosidase and inhibit the release of glucose from
the carbohydrate and hence inhibit the uptake of
postprandial blood glucose. There are many synthetic
drugs available to prevent or treat diabetes such as
acarbose, vogibose and miglitol. However, they may
usually cause hepatic disorders and other negative
gastrointestinal symptoms (Murai et al., 2002).
Hence, antioxidant and α-glucosidase inhibitors from
natural source have become more preferred choice
to prevent or treat diabetes. Traditional medicinal
plants are one of the potential natural sources of these
antioxidant and α-glucosidase inhibitor. Traditional
medicinal plants have been traditionally used by the
society as traditional medicine either via decoction,
tonic, or poultice of whole parts of the plants and
the knowledge is passed down from generation to
generation (Mustafa et al., 2010). Recently, many of
the traditional medicinal plants have gained attention
worldwide due to their potential beneficial usage.
Malaysia is one of the 12 megadiversity countries
of the world. With the diverse species of flora, it is
expected that there are valuable medicinal plants in
the tropical rainforest in Malaysia. The environmental
variations in the growing area of the plants are
known to affect the metabolism of the plants. Such
variations include fluctuations in sunlight, water
stress, temperature, intensity of rain, restrictions on
nutritive components and air humidity (Ahmed et al.,
2012). Variations in these parameters affect both the
primary and secondary metabolism of the plant and
hence giving to the different bioactivity of the plants
(Hong et al., 2008).
Due to the great diversity of floral species in
Malaysia, it is worthwhile to select the local traditional
medicinal plants for the purpose of studying their
antioxidant and antidiabetic activities. Therefore,
in the present study, five local traditional medicinal
plants have been chosen for this purpose. The aims
of this study were to evaluate the antioxidant and
antidiabetic activities of leaves and stems of five
selected traditional medicinal plants using diphenyl1-picrylhydrazyl (DPPH) and α-glucosidase
inhibition assays, respectively and to determine the
effect of location on the bioactivity and efficiency of
plant. The five studied traditional medicinal plants
include Mitragyna speciosa (ketum), Clinacanthus
nutans (belalai gajah), Strobilanthes crispus (pecah
beling), Neptunia oleracea (tangki) and Mentha
asiatica (pudina).
Material and Methods
Plant materials
The plant materials were obtained from Pusat
Pembangunan Komoditi Sendayan, Seremban,
Negeri Sembilan and University Agriculture Park
(TPU), UPM. The leaves and stems were cut into
small pieces and dried in ventilated drying oven at
40°C for 24 hours. They were then ground to a fine
powder (particle size 0.70 mm) and stored in air-tight
containers until further use.
Chemical reagents
Absolute methanol, sodium carbonate, FolinCiocalteu reagent, gallic acid, acarbose, quercetin,
phosphate buffer, α-glucosidase enzyme, glycine and
2,2-diphenyl-1-picrylhydrazyl (DPPH) were supplied
by Merck (Darmstadt, Germany) while p-nitrophenylα-D-glucopyranose (PNPG) was supplied by Sigma
(Aldrich, Germany).
Plant extraction
After the grinding step, the leaves and stems were
extracted using absolute methanol. The extraction
was performed by weighing 20 g of the ground leaves
and stems, and then soaked in 500 mL of absolute
methanol and subjected to sonication (at controlled
temperature) in sonicator (Nexul Ultrasonic Cleaner,
NXP 1002) for 1 h. The mixture was then filtered
using filter paper before concentrated using rotary
evaporator to yield the crude extract. The crude
extracts were stored at 4°C until further analysis.
Total phenolic content
The TPC was determined using the FolinCiocalteu method according to the procedure reported
by Zhang et al. (2006) with some modifications.
Briefly, a volume of 20 µL of standards or test
samples was mixed with 100 µL of Folin-Ciocalteu
reagent in 96-well plates. After 5 minutes, 80 µL of
7.5% sodium carbonates was added to each well.
The plate was then covered and kept in the dark for
30 minutes before the absorbance was measured at
765 nm using a micro-plate reader (SPECTRAmax
PLUS). The analysis was performed in triplicate. The
standard curve was obtained for the calculation of
phenolic content of gallic acid and the results were
expressed in mg GAE/g dry weight basis of sample.
2,2-Diphenyl-1-picrylhydrazyl (DPPH) assay
The assay was conducted as described by Li and
Seeram (2010) and Wan et al. (2012). The assay
Lee et al./IFRJ 21(1): 165-172
was performed by using 96-well microplate. A
volume of 50 µl aliquots of 8 serial dilutions of the
tested samples and quercetin (positive control) with
triplicate was put in the well. After this, 100 µl DPPH
(80 mg/L) was added to each well. The mixture
was then incubated in the dark for 30 minutes. The
absorbance was determined after 30 min of reaction
in the dark at 517 nm with a micro-plate reader
(SPECTRAmax PLUS). The scavenging capacity
(SC) was calculated as SC % = [(Ao-As)/Ao] x 100
where Ao is the absorbance of the reagent blank and
As is the absorbance of the test samples. All tests were
performed in triplicate. The results expressed in IC50
values, which denote the concentration of sample
required to scavenge 50% DPPH free radicals.
α-Glucosidase inhibition assay
The assay of α-glucosidase inhibition activity
performed as described by Kim et al. (2000)
and Sarmadi et al. (2012). ρ-Nitrophenyl-ρ-Dglucopyranosidase (PNPG), was used as substrate
and prepared by dissolving in 50 mM phosphate
buffer (pH 6.5), which is comparable to the condition
of intestinal fluid. The leaves and stems extracts were
prepared at 5000 ppm and 6 serial dilutions were
performed.
The extracts were mixed in the 96-well microplate
and incubated at room temperature for 5 minutes.
Then, 75 µL of PNPG was added to each well of
sample, blank substrate, negative control and positive
control while the rest were loaded with 75 µL of 30
mM phosphate buffer. The mixtures were incubated
for 15 minutes at room temperature. The reaction
mixtures were stop by using stopping agent, 50 µL of
2M glycine (pH 10) for sample, blank substrate and
negative control. The remains were added with 50 µL
of deionized water. Then the absorbance readings were
measured using spectrophotometer (SPECTRAmax
PLUS) at wavelength of 405 nm. The α-glucosidase
inhibition activity of the test sample was expressed
as percentage (%) of inhibition and can be calculated
as % inhibition of sample = [(an-as)/an] x 100% where
an is the difference in absorbance of the negative
control and all the blanks while as the difference in
absorbance of the sample and all the blanks.
Statistical analysis
The results were expressed as mean ± standard
deviation of three replicates. ANOVA was used
to execute the analysis of significant difference.
MS Excel and Minitab 14 software (Version 14,
Minitab Inc, State College, PA, USA) were used
for statistical calculation. Pearson correlation test
was also performed using Minitab 14 software. For
167
Pearson correlation and the IC50 was converted to 1/
IC50 to invert the relation between absorbance and the
activity.
Results and Discussion
Total phenolic content of local traditional medicinal
plants
Phenolic compounds are one of the most
important groups of secondary metabolites present in
plants. Phenolic compounds can be characterized by
the possession of at least one aromatic ring carrying
one or more hydroxyl groups. Due to the presence of
these hydrogen-donating hydroxyl groups, phenolic
compounds are known to possess high antioxidant
activity (Michalak, 2006). Generally, the antioxidant
activity of phenolic compounds is mainly depends
on the number and position of hydrogen-donating
hydroxyl groups on the aromatic ring of the phenolic
compounds (Michalak, 2006). Fruits, vegetables, and
medicinal herbs are the common natural sources of
phenolic compounds.
The TPC of the leaf and stem of each of the plants
are shown in Figure 1. The leaf of Neptunia oleracea
contained the highest amount of phenolic compounds,
whereas the leaf of Strobilanthes crispus (Sendayan)
and Clinacanthus nutans possessed the least amount of
phenolic compounds. The TPC of the plant leaf extracts
varied from 2.64 to 40.88 mg GAE/g dry weight
(DW). The leaf of Strobilanthes crispus (Sendayan)
and Clinacanthus nutans contained 2.64 and 2.68 mg
GAE/g DW, respectively, followed by Strobilanthes
crispus (TPU), Mentha asiatica, Mitragyna speciosa
and Neptunia oleracea at 6.52, 11.8, 24.02 and 40.88
mg GAE/g DW, respectively. The same trend can be
observed for the stems extract of the plants. The TPC
of the plant stems extracts varied from 0.055 to 21.21
mg GAE/g DW. The stems of Strobilanthes crispus
(Sendayan) and Clinacanthus nutans contain 0.055
and 0.12 mg GAE/g DW, respectively, followed by
Strobilanthes crispus (TPU), Mitragyna speciosa and
Neptunia oleracea at 0.46, 0.92 and 21.21 mg GAE/g
DW, respectively. The stem of Mentha asiatica was
not analysed due to the small size of the stem and
insufficiency of sample.
It was observed that the leaf and stem of each of the
compared plants had significant difference (P < 0.05)
in the level of TPC. Biosynthesis of secondary plant
metabolites is often goes along with translocation and
storage (Hartmann, 1996). The secondary metabolites
are produced in a specific organ of the plant, such
as leaf and roots as a protective agent in response
to environmental stress. The secondary metabolites
do not accumulate in the site of synthesis for all
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Lee et al./IFRJ 21(1): 165-172
Figure 1. Total phenolic content of leaf and stem of
local traditional medicinal plants The first subscript
letter is to compare the same part of every plant. The
second subscript letter is to compare between parts of
the same plant. Mean with different subscript letters are
significantly different (P < 0.05).
Figure 2. DPPH radical scavenging activity of leaf and
stem of local traditional medicinal plants.
The first subscript letter is to compare the same part of
every plant. The second subscript letter is to compare
between parts of the same plant. Mean with different
subscript letters are significantly different (P < 0.05).
the time and will transfer to their preferred site of
storage through phloem or xylem tissues (Hartmann,
1996). In this study, the leaves of the studied plants
had higher amount of phenolic compounds than their
stems. This could be explained by the reason that
leaves are the site of biosynthesis of these phenolic
compounds and they moved from leaves to the site of
storage via the phloem or xylem tissues through long
distance translocation. Besides, even the different
parts of the fruits Mangifera pajang and Artocarpus
odoratissimus studied by Abu Bakar et al. (2009)
demonstrated different levels of the bioactivities and
the chemical compounds present. Therefore it is not
surprising for the stem and leaf of the plants in the
present study to possess different level of phenolic
compounds. The difference in TPC between different
parts of Strobilanthes crispus had also been reported
before by Ismail et al. (2012).
It is also interesting to note that there was different
level of TPC in the same plant species collected from
different location. Strobilanthes crispus collected
from Sendayan and TPU. The plants grew at different
location hence were subjected to different level of
stressors. Stressors such as elevated temperature,
drought stress, high irradiation, biological stress,
soil composition and nutrition deficiency result
in synthesis and accumulation of secondary plant
products (Selmar and Kleinwächter, 2013).
In this study, variation between the TPC in
the same plants with those reported by previous
authors was observed. For instance, the TPC of
Neptunia oleracea determined was around 42.88
mg GAE/g DW for leaf and 21.21 mg GAE/g DW
for stem. A lower TPC (4.16 and 4.84 mg GAE/g
DW, respectively) was reported by Rattanasena
(2012) while a higher TPC (42.98 mg GAE/g DW)
was reported by Daduang et al. (2011). Besides, the
different TPC for the Strobilanthes crispus leaves
was reported by Qader et al. (2011) and Ismail et al.
(2012). These differences may be contributed by the
differences in solvent and method of extraction, parts
of plant being analyzed or even climatic condition
and cultural practices, maturity at harvest and storage
condition of the plants.
Free radical scavenging activity of the plant extracts
Antioxidants are able to stabilize or deactivate
the free radicals before they cause oxidative damage
to the cellular structures. Free radical scavenging is
by donating hydrogen to the free radical. Thus, free
radical scavenging activity can be used for determining
the antioxidant capacity of plant extract. In this study,
DPPH free radical scavenging activity of plant leaf
and stem extracts were evaluated to determine their
antioxidant properties. The free radical scavenging
activity was expressed as IC50 value. IC50 is defined
as the concentration of the antioxidant required to
scavenge 50% of the DPPH radical.
Results of free radical scavenging activities
of the leaves and stems of each plant, expressed as
IC50 values are presented in Figure 2. The results
showed that most of the studied plants exhibited
potential antioxidant activity and the IC50 values of
the leaves varied from 35.45 to 1126.63 µg/mL. The
lower the IC50 value, the greater is the free radical
scavenging capability of the plant extract. This is
because a lower concentration is required for the
antioxidant compounds to exhibit 50% of inhibition
on free radicals. Results of the present study showed
that leaf of Neptunia oleracea possess the most
potent free radical scavenging activity with IC50 of
35.45 µg/mL, followed by the leaves of Strobilanthes
crispus (TPU), Mentha asiatica, Mitragyna speciosa
and Strobilanthes crispus (Sendayan) with IC50 of
67.63, 115.9, 127.05 and 445.36 µg/mL, respectively.
Clinacanthus nutans leaf possessed the lowest free
radical scavenging activity with IC50 of 1126.63 µg/
mL. There was significant difference (P < 0.05) in the
Lee et al./IFRJ 21(1): 165-172
free radical scavenging activity of all the plant leaves,
except Mentha asiatica and Mitragyna speciosa. The
results of the stems also showed the same trend.
When comparing the leaf and stem of the same
plant, the same pattern as the results of TPC can be
seen. it was observed that the leaves and stem of the
compared plants had significant difference (P < 0.05)
in the inhibition on free radicals, except Neptunia
oleracea, which showed no significant difference (P
> 0.05) between the two parts. Both the leaf and stem
of Neptunia oleracea had the comparable antioxidant
properties and were the most active plants among
all the compared plants. Since it was found that the
Neptunia oleracea was the best for its high TPC in
both the leaf and stem, it is can be suggested that
phenolic compounds may be the major source of
antioxidant capacity of the plant (Qader et al., 2011).
The TPC and the antioxidant activity were positively
correlated with R value of 78.9%. For all the other
plants, the leaves exhibited higher antioxidant activity
than the stems. It showed that the leaves possess
higher amount of antioxidant compounds than the
stem. It can be explained that the leaves are the site
of biosynthesis of these antioxidant compounds and
these synthesized compounds are translocated from
site of biosynthesis to the preferred site of storage via
xylem or phloem in the stem (Hartmann, 1996).
On the other hand, it was observed that the
Strobilanthes crispus collected from Sendayan
and TPU were significantly different in their free
radical scavenging activities. The difference in the
antioxidant activities can occur due to the different
type and amount of antioxidant compounds present
in the plants. The different type and amount of
antioxidant compounds present in the plants can be
affected by the environmental condition. Factors
such as deficiency of nutrients in the soil, increased
intensity of sunlight, pest infestation, and drought
stress will increase the synthesis and accumulation
of secondary products in the plant (Selmar and
Kleinwächter, 2013). Different stages of growth of
the plants will also have the effect on the bioactive
compounds and bioactivity of the plants (Ismail et
al., 2012).
The results obtained from this study showed some
disagreement with those previous reported by others.
For instance, Parthasarathy et al. (2009) reported
a greater DPPH free radical scavenging activity
for Mitragyna speciosa leaf with IC50 of 37.08 µg/
mL. A lower DPPH free radical scavenging activity
for Neptunia oleracea and Strobilanthes crispus
were reported by Rattanasena (2012) and Qader et
al. (2011), respectively. These differences can be
justified by the variation in solvent and method of
169
extraction used, parts of plant being analyzed, or even
climatic condition and cultural practices, maturity at
harvest and storage condition of the plants.
α-Glucosidase inhibitory activity of the plant
extracts
The α-glucosidase enzyme is one of the key
enzymes involved in dietary carbohydrate digestion
in human. It hydrolyzes the carbohydrate, releasing
glucose and cause the raised postprandial blood
glucose level. Inhibitions to this enzyme can
effectively descent the postprandial blood glucose
level. This is especially beneficial for diabetic
patients. There are many synthetic drugs available to
prevent or treat diabetes such as acarbose, vogibose
and miglitol. However, they usually can cause
hepatic disorders and other negative gastrointestinal
symptoms (Murai et al., 2002). Hence, α-glucosidase
inhibitors from natural source have become the more
preferable as means to prevent or treat diabetes. In
this study, the α-glucosidase inhibitory activity of the
plant leaf and stem extracts were evaluated and the
results were shown in Table 1.
The results showed that for both the leaf and
stem of Neptunia oleracea possessed the most potent
α-glucosidase inhibitory activity. For the leaf, the
inhibitory on α-glucosidase by Neptunia oleracea
was as high as 84.16% at concentration of 39.06 µg/
mL. Meanwhile, Strobilanthes crispus (Sendayan),
Mitragyna speciosa, Strobilanthes crispus (TPU),
Clinacanthus nutans and Mentha asiatica only
showed inhibition values of 46.69, 20.83, 14.02,
13.57 and 9.79%, respectively at concentration of
5000 µg/mL. In contrast to the results of TPC and
DPPH radical scavenging activity, the trend for the
results of α-glucosidase inhibitory activity in stems
were not the same as those of leaf. The inhibitory on
α-glucosidase by Neptunia oleracea stem was as high
as 90.66% at concentration of 39.06 µg/mL, followed
by Mitragyna speciosa, Clinacanthus nutans,
Strobilanthes crispus (TPU) and Strobilanthes crispus
(Sendayan) with percent inhibition of 29.64, 17.67,
14.44 and 0.91%, respectively at concentration of
5000 µg/mL. As shown in the table 1, only the IC50 of
Neptunia oleracea leaf and stem can be determined.
There was significant difference among all the studied
plants by comparing both the leaf and stem.
The high inhibition activity of Neptunia oleracea
on α-glucosidase might be contributed by the high
content of phenolic compounds in the plant as a
remarkably high content of phenolic compounds was
also reported in this study. The phenolic compounds
might be the contributor to α-glucosidase inhibitory
activity. In this study, the TPC and the α-glucosidase
Lee et al./IFRJ 21(1): 165-172
170
Table 1. Percentage of α-glucosidase inhibition and the
IC50 values
Plant
Mentha asiatica
Mitragyna speciosa
Strobilanthes crispus
(Sendayan)
Clinacanthus nutans
Neptunia oleracea
Strobilanthes crispus
(TPU)
Leaf
Leaf
Stem
Leaf
Stem
Leaf
Stem
Leaf
Stem
Leaf
Stem
Highest concentration ( µg/mL )
5000
5000
5000
5000
5000
5000
5000
39.06
39.06
5000
5000
Percentage of Inhibition
9.79±1.0 a
20.83±0.5 ba
29.64±0.6 ab
49.69±1.7 ca
0.91±0.06 bb
13.57±1.2 da
17.67±1.9 cb
84.16±8.8 ea
90.66±1.4 db
14.02±0.3 da
14.44±0.04 ea
IC50 Values ( µg/mL )
ND
ND
ND
ND
ND
ND
ND
19.09±3.2
19.74±1.4
ND
ND
Values are the means ± standard deviation of three replicates.
The first subscript letter is to compare the same part of every plant. The second
subscript letter is to compare between parts of the same plant. Mean with
different subscript letters are significantly different (P < 0.05).
ND = not determined.
inhibition activity were positively correlated with
R value of 67.4%. There was also strong positive
correlation between antioxidant and α-glucosidase
inhibition activities with R value of 81.9%. The
positive relationships between the quantity of
phenolic compounds and α-glucosidase inhibitory
activity of 28 edible plants in Vietnam had been
reported by Mai et al. (2007). This positive relationship
might be found in Neptunia oleracea in this study as
well. Besides, several flavonoids, including luteolin,
amentoflavone, luteolin 7-O-glucoside and daidzein
had been reported to exhibit α-glucosidase inhibition
(Kim et al., 2000). Nonetheless, Tunsaringkarn et al.
(2009) reported a 0% α-glucosidase inhibition for
the Neptunia oleracea extract at a concentration of
1 mg/ml. That was a great difference between the
results obtained from this study and their results.
This difference in the activity can arise because of
the usage of different extraction solvent and method
of extraction
When comparing the leaf and stem of the same
plant, the same as the results of TPC and free radical
scavenging activity , it was observed that the leaves
and stem of the compared plants had significant
difference (P < 0.05) in the inhibition α-glucosidase,
except Strobilanthes crispus (TPU), which showed
no significant difference (P > 0.05) between the two
parts. Unlike the results of TPC and DPPH radical
scavenging activity, stem of some plants showed
greater inhibition activity on α-glucosidase than
their leaves. It was expected that there were other
compounds besides phenolic compounds present in
the stems of these plant that possess α-glucosidase
inhibition activity. The type of alpha-glucosidase
inhibitor drugs available today, such as acarbose
and myglitol are carbohydrate compound. Hence,
besides phenolic compounds, it was predicted that
the compounds present in the stem of the plants could
be some carbohydrates that are act as competitive
inhibitor of alpha-glucosidase enzyme (Sugiwati et
al., 2009).
Strobilanthes crispus collected from Sendayan
and
TPU exhibited different activities on
α-glucosidase inhibition. Due to the different
environmental condition, the type and amount of
secondary metabolites can be different in the plants
(Selmar and Kleinwächter, 2013). The Strobilanthes
crispus from Sendayan showed greater α-glucosidase
inhibitory activity instead of that collected from TPU.
This result was different with those of TPC and DPPH
radical scavenging activity which demonstrated better
activity in Strobilanthes crispus collected from TPU.
Although the TPC in Strobilanthes crispus collected
from TPU was reported to be higher than that from
Sendayan, the type and quality of phenolic compounds
might be different. The phenolic compounds found in
Strobilanthes crispus collected from TPU might be
effective in antioxidant activity but not in inhibition
of α-glucosidase.
Conclusion
The results of the current study showed that
leaves and stems of the five plants tested consisted
of wide range of phenolics compounds and exhibit
potential antioxidant and α-glucosidase inhibitory
activities. The leaves and stems of some of the
plants were different in their bioactivities. The study
revealed that phenolic compounds could be the main
contributors to the antioxidant and α-glucosidase
inhibition activities with R values of 78.9 and 67.4%,
respectively. It was also shown that plants from
different locations exhibited different bioactivities.
It is encouraging to note that Neptunia oleracea
possess remarkable phenolics content, antioxidant
and antidiabetic activities. The results suggested that
phenolic compounds are the major contributor to the
antioxidant and α-glucosidase inhibitory capabilities
of this plant. Consequently, this plant can be
suggested as a potential natural source of antioxidant
and antidiabetic compounds for the prevention or the
treatment of diabetes and its complications. However,
the use of this plant as an alternative remedy for
diabetes requires more extensive studies or the
optimum extraction and isolation of the bioactive
compounds and its safety and efficacy evaluation on
human subjects.
Acknowldgements
The authors wish to thank Universiti Putra
Malaysia for the support and facilities in conducting
this research. We are also expressing gratitude to the
Mrs Zaleha, staff of University Agricultural Park and
Pusat Pembangunan Komoditi Sendayan for their
assistances in obtaining the samples. Appreciation is
Lee et al./IFRJ 21(1): 165-172
also given to Mr. Shamsul Khamis, a resident botanist
at Institute of Bioscience, Universiti Putra Malaysia,
who helps to identify the plant samples.
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